Has anyone ever seen a canon definition of what the 'Fleet Captain' title means? I've always assumed it was a title rather than an actual rank, that denoted which captain would assume overall command in a scenario with multiple ships but no one with actual flag rank. Garth would be a classic example, and Picard as well in the 24th Century.
How Pike meets Kirk while promoting himself to Fleet Captain all in one day

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[Farragut bridge]
Kirk: Report!
Conn officer: Main power is off-line. We've lost shields and our weapons have gone.
Kirk: Perhaps today is a good day to die. Prepare for ramming speed!
Conn officer: Sir, there's another starship coming in. ...It's the Enterprise!
[Enterprise bridge]
Number One: The Farragut's losing life support.
Pike: Bridge to transporter room three. Beam the Farragut survivors aboard.
Number One: Captain, Admiral April's ship has been destroyed.
Pike: What is the status of the Abronath swarm ship?
Spock: It has sustained heavy damage to its outer hull. I am reading fluctuations in their power grid.
Pike: On screen. ...Ensign Christina, open a channel to the fleet.
Christina: Channel open.
Pike: This is Captain Pike of the Enterprise. I am taking command of the fleet. Target all of your weapons onto the following coordinates. ...Fire on my command.
Spock: Sir, the coordinates you have indicated do not appear to be a vital system.
Pike: Trust me Spock, we both know I'm not going to die today.